Push-pull mixing circuit arrangement



Sept. 2, 1952 A. VAN WEEL 2,

PUSH-PULL MIXING CIRCUIT ARRANGEMENT I Filed Sept. 10, 1946 2 SHEETS-SHEET IN VEN TOR.

ADLBERT vm we L M AGENT Sept. 2, 1952 A. vAN,wE:EL 2,609,495

PUSH-PULL MIXING cmcuxw ARRANGEMENT Filed Sept. 10, 1946 -2 SHEETS-SHEET 2 INVENTOR. A051. BERT 4N WEEL Patented Sept. 2, 1952 PUSH-PULL MIXING CIRCUIT ARRANGEMENT Adelbert van Weel, Eindhoven, Netherlands, as-

signor to Hartford National Bankand Trust Company, Hartford, Conn., as trustee Application September 10, 1946, Serial No. 695,888 In the Netherlands May 13, 1943 Section 1, Public Law 690, August s, 1946 Patent expires May 13, 1963 This invention relates to-a mixing circuit arrangement in which the incoming oscillations are-supplied in push-pull arrangement and the local oscillations in single phase (in the same phase) to the input electrodes of two discharge systems each of which comprises at least one cathode, one input electrode and one output electrode.

For an understanding of the invention reference is had to the following detailed description thereof to be read in conjunction with the accompanying drawing wherein: I

Fig. 1 is a schematic circuit diagram of a prior art push-pull mixing system,

- Fig. 2 and 3 are substitution diagrams for the input circuit of the system in Fig. 1, I

Fig. 4 is a schematic circuit diagram of a first preferred embodiment of a mixing system in accordance with the present invention, Fig.5 is a schematic circuit diagram of a second preferred embodiment of the invention, and Fig. 6 is a schematic circuit diagram of a third preferred embodiment of the invention.

Fig. 1 of the accompanying drawing represents a mixing circuit of this kind, which forms part i .capacities between the inductance coils L1 and L1" respectively and the antenna coupling coil Lo are represented by Co and Co" respectively. The connecting point of coils L1 and L1" is earthed through a condenser C2 and connected through a resistance R1 to a supply (not represented) by which the control-grids of the triods -T and T" are suitably biassed. The input capacities of the triodes T'and T" are denoted by C3 and Cs" respectively. p

The cathodes of the triodes T1.,and T are jointly connected to earth through an inductance L2 which is coupled with a coil L3 inserted in the output circuit of a diagrammatically represented local oscillator O. Consequently by means of coil L2 the local oscillations are supplied by means of so-called cathode injection to theinput circuits of the triodes in such a manner that the local oscillations appear in the same phase, at the control-grids of the two triodes.

' A'resonant circuit tuned to the intermediate 7 Claims. (01. 250-20) 2 frequency and consisting of two inductances L4 and L4" and two condensers C4, C4 is connected in push-pull to the anodes of the two triodes i In series with coils L4 and L4" are provided high freque'ncy chokes L and La. which serve to prevent a transmission of the incoming high: frequency oscillations to the receiver parts next to the mixing stage. The connecting pointiof coils L4 and L4" is connected with earth through a condenser C5 constitutingpractically a shortcircuit in regard to the intermediate frequency oscillations, and to the positive terminal, of a source of anode potential (not represented) through a resistance R2. Furthermore coils L42 and L4" ,are inductively coupled with a. second intermediate-frequency circuit Ls, Ce, from which the output voltage of the circuit is taken. The junction of condensers C4 and C4" is connected to the cathodes of the two triodes in order to obtainan electric circuit having a low impedance in regard to anode currents having the oscillator frequency. Since the aforesaid circuit is par. ticularly suitable for frequency-transformation of ultra-short waves, the input resonant circuit L1", L1", C1 will usually be realized in the form of a Lecher system in practice. y

The present circuit exhibits the drawback that the input impedance of the triodes, represented by the capacity C3 and C3" respectively in the drawing, notably in receiving ultra-short'waves has only a small value, due to which the voltage having the oscillator-frequency andbeingset up between the control-grid and the cathodeof any of thetubes, usually becomes too small. To sup;- ply as high as possible a voltage having theoscillator-frequency to the control-grids of the mixing triodes my copending United States Patent application No. 680,930, filed July 2, 1946, suggests to tune the single-phase circuit constituted by the' pair-wise parallel-connectedimpedances of the two halves of the push-pullcircuit (L1', L1 and the parallel-connected input impedances of the two discharge systems (03', C3") jointly with the impedances (L2, C2 common to the input circuits of the two systems, to the frequency of the local oscillations. In-the present case the expression push-pull circuit is to be understood to mean the assembly of impedances available between two not directly interconnected input electrodes of the discharge systems, in which are flowing currents having the frequency of the. incoming oscillationsffthe expression input circuit of a discharge, system. being understood to 'meanallthe impedances available between the input electrodes ofa'dlscharge system and in which are circulating currents associated with this system and having the frequency of the local oscillations associated with this system. Thepush-pull circuit is generally tuned tofithe frequency of the incoming oscillations.

The effect of the aforesaid expedient may be.

explained with reference to Figs. Zand 3.

Fig. 2 represents the substitution diagram of the input circuits of the two mixing triodes. From this diagram it appears that the elements. L1,

L1", Ca, Ca" constitute a Wheatstone bridge in.

regard to the local oscillations. When this bridge is in equilibrium the elements Lo and Clare currentless, so that each of them can. be replaced by a short-circuit. Thus ensues the substitution diagram represented in Fig. 3 where Ll represents the parallel connection of L1 and L1", C'cthe parallel-connection of Co and C", C3 the parallel-connection of C3 and Ca" and D the parallel-connection of the twodipole halves. D and D". Itwilln'ow be readily appreciated that the voltage, having the oscillator frequency set up at the .capacityvcs, i. e. the voltage betweemthe contr'ol g'rid. and. cathode of. each triode is a maxi: mumwhen series-resonanceoccurs in the circuit L1, C2,,L2', C3. The circuit in question must consequently bev tuned to the oscillator frequency. By tuning, moreover, thecircuitsLi, C2 andLz, C3 eachpindividually to the oscillator frequency the radiation of the local oscillations can bereduced to'a minimum by the antenna D.- In practicethe inductance of coil L2 and the capacity of condenser C2 may bemade variable for this purpose and these elements may be adjusted in such a manner that, the radiation of the local oscillations by the antenna is-a minimum and the voltage having the oscillator frequency between the control grid andthe-cathode of each; triode is a maximum.

The invention; is based on the realization that the aforesaid expedient--the tuningof the said circuit to: the frequency of the local,oscillations does-not lead completely to the result aimed at under all conditions, and this because the said single-phase circuit. is usually damped. too strongly. In, addition to the input damping, of the mixing tubes thisdamping is also to be ascribedto a negative backcoupling brought about by the capacitative anode load. In fact, the capacityof condensers C4 and 041 should not be chosen too high, since otherwise the impedance of the intermediate frequency circuit L4, L4", C4, 04'', would become too low. Hence the condensers C4 and C4" constitute a not negligible impedance in regard to the oscillator frequency, so that between the anode and cathode of any of thetriodes an anode, voltage having the oscillator frequency occurs, which leads in phaseby 90 with respect to the alternating control-grid volt"- age. The said anode voltage involves a current through the anodecontrol-gridcapacity, which is in phase opposition in regard to the alternating control-grid voltage and consequently has a damping effect on the said circuit. It is pointed out in this respect that in a similar manner a damping isalso brought about of the input resonant circuit-L1, L1", C1 tuned tothef-requency of the incoming oscillations.

The invention has for its object an improvement of the circuit according to the above-identified copending applicatiomas a result of which the said drawbacksare avoided. According to the ,inventionuse is maderof a positive backcoupling i'nrregard to the localoscillations, as a. result. of

which the said single-phase circuit is undamped.

A particularly suitable embodiment of the invention is obtained by giving the positive backcouplingsuch .ahigh value as to produce the local oscillations in the saidcircuit. In. this case the distinct oscillator O can be dispensed with, which yields a great simplification of the circuit.

Theback-coupling current or -voltage for the positive back-coupling is taken from a part of the output circuit in which appear both the local and the incoming oscillations, so that both the said'circuitand the input circuit of the arrangement are undamped in regard to the incoming oscillations. The positive backcoupling thus ob- .tained. in regard to the incoming oscillations has the advantage that an improvement of the signalto-noise ratio is obtained.

The damping of said circuit is generally different. fromthe damping of the input circuit for the incoming oscillations, inter alia because the lastmentioned circuit is damped to a much higher degree by the antenna. Therefore it is advisable that the strength of the backcoupling for both kinds of, oscillations'should be madedifferent, which can be ensured by taking an additional back-coupling current or voltage from a part of the, output circuit in which appear eitherexclusively'the local oscillations or exclusively the incoming oscillations.

In a suitable embodiment of the invention the positive back-coupling is obtained by making the impedance of the output circuit for the local oscillations and, if desired, also for the incoming oscillations substantially inductive. To such end, for instance, inductances'may be inserted in parts of the output circuit in which appear both the local oscillations and the incoming oscillations, the'desired intensity difierence-of the back-coupling in regard to-both' kinds of oscillations being obtained by inserting one or more-capacities in a part of the outputcircuitin which appear either exclusively-the local oscillationsor exclusively the incomingoscillations;

The inventionwill be more fully explained by Figs. 4, Sand 6 of the drawing representingembodiments thereof.

The circuit represented in Figure 4 substantially corresponds to that shown in Figure l, but the condenser-C2 and the inductanceLz are ad justable to permit the said circuit to be tuned to theoscillator'frequency in the manner referred to above. Furthermore; according to the invention, the conductor through which the junction of the condensers C4 and C4 is connected to the cathode includes an inductance L1. Since in the said conductor exclusively acurrent having the oscillator frequency is flowing'a positive back-coupling in regard to the local oscillations is obtained as a result thereof. In fact, due to the voltage dropacross the coil L7, an alternating anodevoltagehaving the" oscillator frequency and lagging the alternating control-grid voltage by is set up between the anode and cathode of each of the triodes T and '1'". Consequently currents having the oscillator frequency, which are in phase with the alternatingcontrol-grid voltage and so cause undamping of the said circuit, are passing through the anode/control-grid capacities C7 and C7". This undampingis preferably raised to such a degree that the arrangement tends to-generate oscillations having the oscillator-frequency, in which case the distinct oscillator O'can' be dispensed with..

Figure 5 represents a circuit arrangementin which use is made. at, the same time of aipo'sitive .coming oscillations.

connection of the condensers C4 and C4". the case of complete symmetry of the outputback coupling. in regard or theincoming'os'cilinserted in apart of the output circuit in which appear both the local oscillations and the in- Similarly to what has been said above in regard to the local oscillations with respect to the circuit shown in Figure 4 a posi-- tive back-coupling for the incoming oscillations is now also obtained, .as a result of which the,

input resonant 'circuit'Li', Li, C1 is undamped and at the same: time an increase in the signalto-noiseratio is obtained.

The inductances of coils La and La" are preferably chosen in such a manner that the sigtates as arule a practically complete undamp-' ing of the input resonant circuit L1, L1", Ci. Since the input resonant circuit is comparatively strongly damped by the antenna D, D, whereas the local oscillations are acted upon only to a slight degree by the antenna, the required intensity of the back-coupling in regard to the incoming oscillations is generally greater thanin regard to the local oscillations, so that the backcoupling in regard to the local oscillations, which is caused by the coils Lsf and La", must be decreased. In the arrangement shown in Figure this is effected by providing a condenser Cabetween the junction of the condensers C4 and C4 and the cathodes of the triodes; In this case the condenser C3 is exclusively traversed by a current having the oscillator frequency and consequently reduces the intensity of the backcoupling in regard to the local oscillations without acting on the back-coupling for the inoorning oscillations.

The same efiect can be obtained by connecting a condenser C9 in parallel with the seriescircuit this condenser is exclusively traversed by a current having the frequency Of the incoming oscillations. To maintain the correct tuning of the circuit consisting of inductances L4, L4 and condensers C4, C4", C9 to the intermediatefrequency the capacities of the condensers C4 and C4", after providing the condenser C9, must be reduced by a corresponding amount. This results in an increase of the capacitative impedance of the output-circuit for the local oscillations,-

intensity of the back-coupling in regard to the local oscillations can be made variable.

When it is desirable that the intensity oi! the backcoupling in regard to the incoming oscillations should be variable, the inductances of coils LB and Le may, moreover, be made variable or variable condensers may be connected in series with these coils.

Figure 6 represents a circuit arrangement ac-' cording to the invention, in which the input resonant circuit L1, L1, C1 is provided between the cathodes of the triodes T and T", the grids being jointly earthed across the inductance La. In this case the required reduction in intensity of the backcoupling in regard to the local oscillations can be achieved, similarly to the circuit shown in Figure 5, by providing a condenser Ca in parallel with the series connection of con- I QQCOEQQB den'sers C4 and C4" and by'reducing the capacity between the junction of the condensers C4 and C4" and the cathodes. When the back-coupling in regard to the-local oscillations is to be made stronger than in regard to the incoming oscillations-this may, for instance, be the case when the local oscillations are to be generated by the arrangement itselIthis can be achieved in the circuit shown in Figure 5 by replacing the condenser Ca by an inductance or, in the circuit shown in Figure 6, by replacing the inductance Lo by a condenser.

' Instead of giving the back-coupling in regard to the incoming oscillations and in regard to the local oscillations difierent values, an additional damping resistance may "be inserted in the input circuit for one or both kinds of oscillations. This damping resistance may, for instance, be provided in parallel with the condenser C1 when an additional damping for the incoming oscillations is necessary. When, in contradistinction thereto, an additional damping for the local oscillations is required, a damping resistance in series with the condenser C2 or with the inductance Lz may be provided for this purpose.

What I claim is:

1. An arrangement for mixing an incoming wave with'local oscillat-ions'to produce an intermediate frequency signal, comprising a pair of electron dischargetubes each having'a cathode el'ectrode,"a grid electrode and an anode,'a'first resonant circuitarrangedto apply the incoming wave in push-pull relation to one set or corresponding electrodes in said tubes, an output system including a second resonant circuit coupled to the anodes of said 'tub'e's to derive in push-pull relation the intermediate frequency signal there'- from, a third and series "resonant circuit coupled between the electrical midpoint of said first circuit and the other set of corresponding electrodes to apply local oscillations in cophasal relation to the grid electrodes of said tubes, and means to introduce positive backcoupling between the output system of said tubes and said third resonant circuit.

2. A mixing circuit as defined in claim 1 further characterized in that said means has a value at which the intensity of said positive backcoupling generates local oscillations in said mixing circuit.

3. An arrangement for mixing an incoming wave with local oscillations to produce an intermediate frequency signal, comprising a pair of electron discharge tubes each having a cathode, a grid and an anode, a first resonant circuit arranged to apply the incoming wave in push-pull relation to the grids of said tubes, a second resonant circuit coupled to the anodes of said tubes to derive in push-pull relation the intermediate frequency signal therefrom, a third and series resonant circuit coupled between the electrical midpoint of said first circuit and the cathodes of said tubes to apply local oscillations in cophasal relation to the grids of said tubes, and means to efiect negative damping of said first and third resonant circuits, said means including an inductive impedance interposed between each of said anodes and said second resonant circuit to introduce positive backcoupling between the anode-cathode circuits or said tubes and said first and third resonant circuits.

4. An arrangement for mixing an incoming wave with local oscillations to produce an interrcircuit; 5, An arrangement for mixing an incoming elec mn disc a e u s ch har ner sthode a. rid. and an an de. a firs eson nt; ircui ra ed t a ply-t e n m ns a n P h-p relation to; the grids of 'said tubes-,a second resonant circuit: coupled; to the; anodes of said tubes toiderive-in push-pull relation the intermediate radio wave; with local oscillations to produce an interm diate q enc is-Hal qm r ne a pai of; electron discharge tubes eachhaving a cathode, a d a an. ne a first e f le +re m nant circuit including a center tapped; inductance connected between said grids, said first circuit being; tuned to the incoming wave; to apnly same in push-pull relation to said; grid-s, a; second parallel-resonant-circuit' constituted by an inductor in parallel with a pair-of series connected condensers connected between the anodesof said tubes and tuned to the intermediate irequency radio frequency cholges interposed between the ends of said inductor and said condensers, a series-resonantcircuit connected be- Q tween the tap in said inductance and the cathodes of said tubesand tuned to the frequencycf local oscillations to app y same in cophasal relation to said grids, inductive feedback impedances interposed between eachanode and each choke to effect; negative. damping of said first parallelresonant circuit and; saidseries-resonant circuit, 'and' a capacitor connected; between the junction of said series connected condensers andthe cathodes. of said tubesto reduce the negative damping ofsaid series-resonant circuit.

6. An arrangement for mixing an incoming radio wave with local, oscillations to produce an mediate ir ueccr deca m rises a pair 1 n ermedia e, re uent? signal comprising anei of e ect on i h r tub ea h havin sca hode, agrid andan anode, a first parallel-resonant circuit includinga center-tapped inductanceconnected between said cathodes, said first circuit being; tuned to the incoming wave to apply same in push-pull relation, to said cathodes, a second parallel-resonant circuit constituted by an inductor in parallel with a pair of series connected condensers connected. between the anodes of said tubes and tuned to, the intermediate frequency,

7 radio frequency chokes interposed between the ends of said inductor and, said. condensers, a series-resonant circuitconnectedbctween the tap in said inductance and the grids of said tubes and tuned to the frequency of local oscillations to apply same cophasalrelation to said grids, inductive feedback impedances. interposed between each andoeand each choketo efifect negative damping of saidfirst parallel-resonant circuit and said series-resonant circuit, andanadditional inductive feedback impedance connected betweenthe junction of said series-connected condensers and the. grids of said tubes. to increase the negativedampingof said series resonant circuitto. the point resulting in the generation of local oscillations,

7. An arrangement as set forth in claim 6 furtherincluding a variable condenser in shunt relation withv the series connected condensers to control the negative damping of said series,- resonant circuit.

. ADELBERT VAN WEEL.

REFERENCES CITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Number Hansell Oct. 4, 1949 

